Crystal structures of influenza A virus matrix protein M1: variations on a theme.

Martin K Safo,Qibing Zhou,Faik N Musayev,Umesh R Desai,Hang Xie,Philip D Mosier,Luis Menéndez-Arias

doi:10.1371/journal.pone.0109510

Abstract

Matrix protein 1 (M1) of the influenza A virus plays multiple roles in virion assembly and infection. Interest in the pH dependence of M1's multiple functions led us to study the effect of subtle pH changes on M1 structure, resulting in the elucidation of a unique low-pH crystal structure of the N1-165-domain of A/WSN/33 (H1N1) M1 that has never been reported. Although the 2.2 Å crystal structure of M1 N-terminus shows a dimer with the two monomers interacting in a face-to-face fashion at low pH as observed earlier, a 44° rotation of the second monomer has led to a significantly different dimer interface that possibly affects dimer stability. More importantly, while one of the monomers is fully defined, the N-terminal half of the second monomer shows considerable disorder that appears inherent in the protein and is potentially physiologically relevant. Such disorder has not been observed in any other previously reported structure at either low or high pH conditions, despite similar crystallization pH conditions. By comparing our novel N1-165-domain structure with other low-pH or neutral-pH M1 structures, it appears that M1 can energetically access different monomer and dimer conformations, as well as oligomeric states, with varying degree of similarities. The study reported here provides further insights into M1 oligomerization that may be essential for viral propagation and infectivity.

Highlights

The matrix protein 1 (M1) of the influenza A virus is a 252amino acid protein [1], comprised of an N-terminal domain (165 amino acids; N1–165-domain) and a C-terminal domain (87 amino acids; C166–252-domain), and plays an essential role in structural integrity, replication and budding of the virus
M1 directly interacts with viral ribonucleoprotein consisting of viral RNA, RNA polymerases and RNAbinding nucleoprotein (NP) and mediates the import of vRNP for vRNA synthesis in the nucleus of infected cells
Despite both structures showing identical monomer folds consisting of two 4-helix bundles that are connected by another helix-containing segment, the low-pH structures show a dimeric structure in contrast to the loosely arranged monomeric structures with very different monomer– monomer arrangements observed at neutral pH, resulting in significantly different models proposed for M1 oligomerization and assembly [12,14]

Summary

Introduction

The matrix protein 1 (M1) of the influenza A virus is a 252amino acid protein [1], comprised of an N-terminal domain (165 amino acids; N1–165-domain) and a C-terminal domain (87 amino acids; C166–252-domain), and plays an essential role in structural integrity, replication and budding of the virus. Previous structural studies on the truncated N1–165-domain have focused on either low pH (pH,4.5) [4,12], a condition similar to endosome acidification after virus particles are internalized [13], or neutral pH resembling the cytoplasmic environment at which virus particles form [14,15]. Despite both structures showing identical monomer folds consisting of two 4-helix bundles that are connected by another helix-containing segment, the low-pH structures show a dimeric structure in contrast to the loosely arranged monomeric structures with very different monomer– monomer arrangements observed at neutral pH, resulting in significantly different models proposed for M1 oligomerization and assembly [12,14]

Methods

Results

Discussion

Conclusion